Why can't anyone just answer the question that an OP asks? It always seems to turn into a discourse that veers off course.
Geez.
So...the real question is: "How much space does Win8 take?" Doesn't matter what size SSD you have, Win8 and Win programs will take a certain amount of space. Windows' needs don't change based on whether you have a 128g or 256g drive. They require what they require. You either will have enough space or you won't and you'll have to make compromises.
Windows 8 itself takes about 8.3 gigs on a drive. That's a typical installation. That's just Windows and nothing else.
Any other program you run will take additional space.
Therefore, depending on what you want/plan to run, you may need to allocate 60 - 100 gigs of SSD space devoted to win8 and all the programs you need/want to run. If all you want to run are a few games and a couple Win8 programs, then maybe you can get away with a partition size of 40 to 50 gigs.
Can you accomplish this with a 128gig drive? You probably can, but you'll have limited space and will need to make compromises. You may not be able to run all the games you want. Decide what you really need to run, calculate out the space each program needs + data needs + temp file space, etc.